There are a number of processes for epitaxial growth of various II-VI, III-V, and IV-VI compounds (including mixed compounds). U.S. Pat. No. 4,186,045, which is here incorporated by reference, discloses a method of liquid phase epitaxial growth in which such compounds are grown from a solution by precipitation of the constituents of the solution onto a seed or substrate. In the method there described, the substances of which the material is formed are melted together to provide a liquid solution that includes the constituents of the material, the solution is brought into contact with the appropriate seed or substrate, and the material to be grown is then precipitated from the solution onto the interface between the solution and seed. In the process of the patent, precipitation is effected by electromigration. In other epitaxial growth systems it is caused by other mechanisms, such as Peltier cooling at the seed-solution interface.
In epitaxial growth processes the solution from which growth proceeds must be in equilibrium, i.e., it must be a saturated solution of the constituents of the to-be-grown compound, at the growth temperature. The growth temperature is typically well below the melting point of the compound, the relative percentages of the constituents present in the saturated solution is typically quite different from that in the compound produced, and the solubility of one or more of the constituents in another also varies with temperature.
All of this makes it difficult to provide the necessary equilibrated solution. Typically, it has been necessary to heat the solution out of contact with the seed or substrate, and not to bring the two into contact until equilibrium at the growth temperature has been achieved; or carefully to add one or more constituents to the solution as it is being heated, in an effort continuously to maintain a saturated solution despite continuing changes in the conditions necessary for equilibrium and to avoid undesirable reaction between the solution constituents and the substrate. Neither procedure is satisfactory for automated or relatively high-level production process. The problems presented are particularly acute when, for example, the planned automated growth is to take place in space.